The present invention relates to curable compositions. In particular, this invention relates to a curable composition including an alkoxysilane containing oligomer and a catalyst, a second curable composition including an alkoxysilane containing oligomer and a photocatalyst, and methods of forming cured films.
Polymeric materials are used extensively as films and coatings to protect and to improve the appearance of substrates. To enhance the durability, toughness, and barrier properties of the films and coatings, the polymeric materials are often crosslinked, using a variety of cure chemistries. One such crosslinking chemistry is the condensation reaction of alkoxysilanes. Polymers with alkoxysilane groups, usually in the presence of catalyst, are reactive with moisture and can form crosslinked polymer films at ambient temperatures.
U.S. Pat. No. 4,499,150 teaches a method for coating substrates wherein the coating composition contains an addition interpolymer having alkoxysilane and/or acyloxysilane groups. The crosslinkable interpolymer is formed by the polymerization of alkoxysilane monomers and/or acyloxysilane monomers with other silicon-free monomers. The interpolymer is polymerized in an organic solvent by a batch process at 119xc2x0 C. and uses initiators and chain transfer agents to achieve a desired molecular weight in the range of 2,000 to 20,000.
It is known in the art that high temperature processes are effective at producing low molecular oligomers from ethylenically unsaturated monomers. At high temperatures, depolymerization reactions and chain fragmentation processes compete with the polymerization reactions which grow the polymer chain length. For example, U.S. Pat. No. 5,710,227 teaches a high temperature polymerization of acrylic monomers above 150xc2x0 C. by a continuous process to produce acrylic acid homo-oligomers and co-oligomers. One advantage of a high temperature process is the ability to eliminate the chain transfer agents which are often needed to control the molecular weight of the oligomers produced by lower temperature processes. Drawbacks to the use of chain transfer agents are that they add to the cost of the process, impart unneeded-functionality to the polymer, may introduce salts into the product, or necessitate a product separation step. Also, the mercaptan chain transfer agents commonly employed are not only expensive but require special handling since they are extremely odoriferous. A second advantage is that high temperature polymerization conditions can be readily adapted to a continuous process to obtain high production rates of oligomers.
A curable composition incorporating an oligomer containing alkoxysilane groups wherein the oligomer has been produced by a facile and economical process has long been sought.
In the present invention, a curable composition is provided including a alkoxysilane or acyloxysilane oligomer formed by a continuous reaction process at 150xc2x0 C. to 500xc2x0 C. The high temperature reaction continuous process produces alkoxysilane oligomers with degrees of polymerization ranging from 2 to 100 preferably without the use of chain transfer agents and with low levels of initiators. In an alternate embodiment of this invention, the alkoxysilane oligomer may be produced by the high temperature process without the use of solvents. A curable composition containing an alkoxysilane oligomer and a photoinitiator is also provided whereby the cure is initiated by exposure to actinic radiation.
In the first aspect of this invention, there is provided a curable composition including:
(A) an oligomer prepared by a continuous process from one or more monomers selected from the group consisting of ethylenically unsaturated alkoxysilanes and acyloxysilanes, and optionally, one or more other ethylenically unsaturated monomers, wherein the monomers are polymerized at a temperature of 150xc2x0 C. to 500xc2x0 C., wherein the oligomer has a degree of polymerization from 2 to 100; and (B) a catalyst.
In a second aspect of the present invention, there is provided a method of forming a film incorporating the curable composition of the first aspect.
In the third aspect of the present invention, there is provided a curable composition comprising:
(A) an oligomer comprising moieties selected from the group consisting of alkoxysilane and acyloxysilane moieties wherein the oligomer is prepared from the polymerization of ethylenically unsaturated monomers, wherein the oligomer has a degree of polymerization from 2 to 100; and
(B) a photoinitiator.
In a fourth aspect of the present invention, there is provided a method of forming a film incorporating the curable composition of the third aspect.
As used herein, the term xe2x80x9cacrylatexe2x80x9d refers to esters of acrylic acid and the term xe2x80x9cmethacrylatexe2x80x9d refers to esters of methacrylic acid. As used herein, the term xe2x80x9csubstantially freexe2x80x9d means less than 0.5% by weight. As used herein, the term xe2x80x9coligomerxe2x80x9d refers to a polymer prepared from ethylenically unsaturated monomers with a degree of polymerization in the range of 2 to 100.
The curable compositions of the present invention are believed to cure by covalent bond formation through reactions between the alkoxysilane containing oligomers. In the presence of a catalyst, the alkoxysilane groups can undergo condensation reactions leading to bond formation between oligomer chains and an increase in the molecular weight of the composition. The curing can be as simple as the formation of a single Sixe2x80x94Oxe2x80x94Si between two different oligomer chains or can be as extensive as the formation of a network of Sixe2x80x94Oxe2x80x94Si crosslinks throughout the curable composition.
The oligomer component of the curable composition is an alkoxysilane containing polymer. The oligomers are prepared by polymerizing ethylenically unsaturated alkoxysilane, acyloxysilane monomers or mixtures of these monomers to form oligomers or with other monomers to obtain co-oligomers. The choice of monomers is dependent on many factors including the intended end use of the curable composition, viscosity of the curable composition, and the monomer costs. Suitable alkoxysilane and acyloxysilane monomers include, for example, vinyl alkoxysilanes, allyl alkoxysilanes, acryloxyalkyl alkoxysilanes, methacryloxyalkyl alkoxysilanes, vinyl acyloxysilanes. Preferred vinyl alkoxysilane monomers are vinyl trialkoxysilanes, vinyl monoalkyldialkoxysilanes, and vinyl dialkylmonoalkoxysilanes wherein the alkyl group contains 1 to 6 carbons and the alkoxy group contains 1 to 6 carbons.
Examples of preferred alkoxysilane and acyloxysilane monomers include vinyl trimethoxysilane, vinyl triethoxysilane, vinyl triisopropoxysilane, vinyl triacetoxysilane, vinyl methyldimethoxysilane, vinyl dimethylethoxysilane, allyl trimethoxysilane, 3-acryloxypropyl trimethoxysilane, 3-acryloxypropyl methyldimethoxysilane, methacryloxymethyl triethoxysilane, methacryloxypropyl methyldiethoxysilane, and methacryloxymethyl ris(trimethylsiloxy)silane.
Optional ethylenically unsaturated monomers suitable for copolymerization with the ethylenically unsaturated alkoxysilane and acetoxysilane monomers are esters of acrylates, esters of methacrylates, amides of acrylates, amides of methacrylates, vinyl aromatics, and vinyl esters of carboxylic acids. Examples of ethylenically unsaturated monomers include methyl acrylate, ethyl acrylate, butyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, methyl methacrylate, butyl methacrylate, N,N-dimethylaminopropyl methacrylamide, vinyl acetate, and styrene. Preferred monomers are ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, vinyl acetate, and methyl acrylate.
As used herein, the term xe2x80x9cco-oligomerxe2x80x9d is defined as an oligomer containing an alkoxysilane or acyloxysilane monomer and at least one other monomer. The monomer units of the co-oligomer may be arranged to form alternating, random, or block polymer structures. Oligomers formed from more than two different types of monomers, such as terpolymers or xe2x80x9cter-oligomersxe2x80x9d are also contemplated. In the broadest sense, it is understood that in an oligomer with a degree of polymerization equal to N, the N monomer units can be independently selected such that it would be possible to form an oligomer with as many as N different monomers.
In a co-oligomer sample prepared from an alkoxysilane or acyloxysilane monomer and at least one other monomer, the compositions of the individual oligomer chains may contain various ratios of the two or more monomers including a small fraction of homo-oligomers formed from each of the monomers. The compositions of the oligomers reported herein are the average mole ratios of the individual monomer units contained within the oligomers of the curable composition.
The average compositional range of the oligomers of the curable composition can vary from oligomers composed only of alkoxysilane or acyloxysilane monomer or mixtures of the alkoxysilane and/or acyloxysilane monomers to co-oligomer composed of an average of 1 alkoxysilane or 1 acyloxysilane monomer unit per oligomer chain. A preferred composition range of alkoxysilane or acyloxysilane monomer to other monomer is 1:10 to 4:1 and a more preferred range is 1:6 to 2:1.
Various synthetic methods exist to prepare the oligomers of the curable composition including polymerization from ethylenically unsaturated alkoxysilane and acyloxysilane monomers or mixtures of these monomers with other ethylenically unsaturated monomers. Polymerization methods include anionic polymerization as disclosed in U.S. Pat. No. 4,056,559, radical polymerization in solution or bulk as described in U.S. Pat. No. 5,739,238, radical polymerization with chain transfer agents such as cobalt complexes as described in Polymer Preprints, 1998, Vol 39(2), pp. 459-460 by Steward et al., catalytic chain transfer polymerization with terminally unsaturated oligomers used as chain transfer agents as described in U.S. Pat. No. 5,264,530, high temperature radical polymerizations in batch, stirred tank, or tubular reactors. The polymerization processes can be batch, semicontinuous, or continuous processes. An alternate synthetic method to produce the oligomers of the curable composition is attaching alkoxysilane or acyloxysilane groups onto an existing oligomer as described in U.S. Pat. No. 4,707,515.
A preferred process to prepare the oligomers is continuous polymerization of the unsaturated alkoxysilane or acyloxysilane monomers with other monomers. The first step of this preferred process is forming a reaction mixture containing:
(a) from 0.5 to 99.95% by weight of the reaction mixture of one or more ethylenically unsaturated alkoxysilane and/or acyloxysilane monomer, and optionally, one or more other ethylenically unsaturated monomers;
(b) from 0.05 to 25% by weight, based on the weight of the ethylenically unsaturated monomer, of at least one free-radical initiator;
(c) from 0-99.5% solvent, based on the weight of the reaction mixture.
Preferably, the reaction mixture contains 10% to 99.95% by weight, and most preferably, 50% to 98% by weight, based on the weight of the reaction mixture, of at least one ethylenically unsaturated monomer. Preferably, the reaction mixture contains 0.1% to 5% by weight, and most preferably, 1% to 3% by weight, based on the weight of the ethylenically unsaturated monomer, of at least one free-radical initiator.
The preferred process is suitable for forming oligomers of the ethylenically unsaturated alkoxysilane and/or acyloxysilane monomers, and co-oligomers of the ethylenically unsaturated alkoxysilane and/or acyloxysilane monomers with the other ethylenically unsaturated monomers.
Initiators for carrying out the process of the present invention are any conventional free-radical initiators including, but not limited to, hydrogen peroxide, alkyl hydroperoxides, dialkyl peroxides, peresters, percarbonates, persulfates, peracids, oxygen, ketone peroxides, azo initiators and combinations thereof. Specific examples of some suitable initiators include hydrogen peroxide, oxygen, t-butyl hydroperoxide, di-tertiary butyl peroxide, tertiary-amyl hydroperoxide, methylethyl ketone peroxide, and combinations thereof.
The monomers may be polymerized as a dilute solution in solvent, although the preferred process does not require solvent, nor is the use of solvents preferred. The reaction mixture may contain one or more solvents at a level of from 0% to 99.5% by weight of the reaction mixture, preferably from 0% to 70% by weight of the mixture, and most preferably from 0% to 55% by weight of the reaction mixture. Suitable solvents for the preferred process are capable of dissolving the one or more monomers, especially under the supercritical fluid conditions of the process, and the oligomers formed therefrom. Suitable solvents for the present invention include, for example, ethers such as tetrahydrofuran, ketones such as acetone: esters such as ethyl acetate: alcohols such as methyl alcohol and butyl alcohol; alkanes such hexane and heptane; aromatic hydrocarbons such as benzene, toluene, and xylene; supercritical fluids such as carbon dioxide; and mixtures thereof. Supercritical fluids such as carbon dioxide are particularly useful because the solvent readily is stripped from the product and may be recycled. In the second step of the preferred process, the reaction mixture is continuously passed through a heated zone, wherein the reaction mixture is maintained at a temperature of at least 150xc2x0 C. under elevated pressure. Once the reaction mixture is formed, it is preferable to have the passing reaction mixture reach the polymerization temperature as rapidly as possible. Preferably, the reaction mixture reaches the polymerization temperature within 2 minutes, more preferably within 1 minute, most preferably within 30 seconds. Prior to reaching the reaction temperature, the reaction mixture may be at any suitable temperature, preferably at a temperature of from 20xc2x0 C. to 450xc2x0 C., most preferably from a temperature of 20xc2x0 C. to 60xc2x0 C. The polymerization is conducted at a temperature of at least 150xc2x0 C., and is preferably conducted at a temperature in the range of from 200xc2x0 C. to 500xc2x0 C., and most preferably at a temperature in the range of from 275xc2x0 C. to 450xc2x0 C.
The oligomerization at the elevated temperatures of the preferred process is rapid. Thus, the reaction mixture may be maintained at the polymerization temperature for as little as 0.1 seconds up to 4 minutes, preferably from 0.5 seconds to 2 minutes, most preferably from 1 second to 1 minute.
The elevated temperatures of the polymerization require that the polymerization reactor be equipped to operate at elevated pressures of at least 30 bars to maintain the contents of the reactor as a fluid at the reaction temperature. In general, it is preferred to conduct the polymerization at from 70 bars to 350 bars, and more preferably at from 200 bars to 300 bars.
In the preferred process to produce the oligomers of the present invention, the ethylenically unsaturated monomers, initiator, and, optionally, solvent are combined to form a reaction mixture. The order of combining the components of the reaction mixture is not critical to the process of the present invention. In one embodiment of the present preferred process, it may be desirable to use one or more solvents, heat the one or more solvents to an elevated temperature, and add the one or monomers and the at least one initiator to the heated solvent to form the reaction mixture. It is preferred to add the initiator last. The reaction mixture can be formed at a temperature below, at or above the polymerization temperature.
Reactors suitable to produce the oligomers of the present invention by the preferred process include tubular reactors having moving parts such as internal rotors or reactors having no moving parts. These reactors may have any cross-sectional shape that permit continuous, steady state flow and that may operate under elevated temperatures and pressures. Such reactors are typically made from inert materials, such as stainless steel or titanium. The reactor may be of any length and cross-sectional dimension that permits effective temperature and pressure control.
The preferred process to produce the oligomers of the present invention generally results in a conversion of the monomers into oligomer of from 10% to greater than 95% relative to the initial amount of the one or more monomers present in the reaction mixture. If residual monomer levels in the oligomer mixture are unacceptably high for a particular application, their levels can be reduced by any of several techniques known to those skilled in the art, including rotary or wiped film evaporation, distillation, and vacuum distillation. Preferably, any monomers which may be present in the oligomer are distilled or xe2x80x9cstrippedxe2x80x9d and recycled for later use.
The preferred process to produce the oligomers included in the curable composition of the present invention results in oligomers having low molecular weights and narrow polydispersities. Furthermore, embodiments of the process result in products that do not require the removal of organic solvents (if none were used in the process) and are not contaminated with high levels of salt from initiator fragments, chain transfer agents, or other synthesis adjuvants. The preferred process may be used to produce oligomers having a degree of polymerization in the range of 2 to 100, preferably in the range of 3 to 50, and most preferably in the range of 4 to 25 wherein the degree of polymerization is the number of residues of the ethylenically unsaturated monomer units in an oligomer chain.
The consistency of the products ranges from a thin, water-like fluid to a viscous, taffy-like fluid. Furthermore, they do not require the use of solvents in the preparation or use and are substantially free of contaminants, including salts, surfactants, metals and the like.
Many catalysts are known to cure the alkoxysilane containing oligomers including organic acids such as p-toluenesulfonic acid, trifluoroacetic acid, methane sulfonic acid, trifluoromethane sulfonic acid, and n-butylphosphonic acid, inorganic acids such as phosphoric acid, metal salts of organic acids such as tin naphthenate, tin benzoate, tin octoate, tin butyrate, dibutyltin dilaurate, dibutyltin diacetate, iron stearate, and lead octoate, and organic bases such as isophorone diamine, methylene dianiline, and imidazole. xe2x80x9cCatalystsxe2x80x9d herein include salts which generate a catalyst upon the application of heat. Excluded from xe2x80x9ccatalystsxe2x80x9d herein are photoacids and photobases which require exposure to radiation to generate the active acid or base form. A preferred catalyst is p-toluenesulfonic acid. The catalysts are added at various levels depending upon the intended use and method of application of the curable composition. A low level of catalyst is often chosen to provide sufficient time to allow wetting of the substrate and leveling of the curable composition prior to gelation of the composition. Effective catalyst levels are in the range of 0.01% to 10%, preferably in the range of 0.05% to 5%, and most preferably in the range of 0.1% to 1%, based on the weight of alkoxysilane and/or acyloxysilane containing material in the curable composition. The curable composition including inorganic pigment such as titanium dioxide may be require a higher catalyst level to effect cure than the curable composition without the inorganic pigment. The catalyst may be added as a neat material or diluted in a solvent.
The order of addition of the oligomer and the catalyst is not important. Typically, the smaller volume of catalyst is added to the larger volume of oligomer with sufficient mixing to provide a uniform concentration of catalyst in the curable composition. The curable composition including the oligomer and the catalyst is a stable mixture in the absence of moisture. Alternately, the oligomer and the catalyst can be stored separately and blended immediately prior to use.
The curable composition may optionally contain solvent. The solvent choice and level are typically based on many factors including compatibility with the other components of the curable composition, cost, volatility of the solvent, and desired application viscosity of the curable composition. Various solvents or solvent blends may be employed including alcohols such as methanol, ethanol, and isopropanol, aromatic hydrocarbons such as toluene, xylene, and naphtha, ether alcohols such as ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, propylene glycol monomethyl ester, ketones such as methyl ethyl ketone, methyl isobutyl ketone, and esters such as butyl acetate. It is preferred that the oligomer of the curable composition is fully soluble in the solvent but the oligomer can also be provided as a dispersion. The solids levels of the oligomer of the curable composition may be in the range of 1% to 100%, preferably in a range of 20% to 100%, and most preferably in a range of 40% to 100%, based on the weight of the non-solvent components. In one embodiment, the curable composition may be provided at low viscosities without the addition of solvent wherein the viscosity of the curable coating is below 15 Pascal second, preferably below 5 Pascal second, and most preferably below 1 Pascal second, as measured by a Brookfield DV-I+ viscometer with the LV spindles at 20xc2x0 C.
The curable compositions of the present invention may contain other ingredients including pigments, fillers, fibers, dyes, biocides including mildewcides and fungicides, plasticizers, humectants, adhesion promoters, surfactants, wetting agents, and flow adjuvants to modify the rheology and flow. Further, the curable compositions may be modified by the addition of other polymers including dispersion, emulsion, and solution polymers. These polymers can be nonreactive or may be functionalized with various reactive groups to provide a second means of curing the curable composition, for example, the reaction between alcohol groups and isocyanates to form urethane linkages.
Various techniques can be employed to apply the curable composition to substrates including spraying, dipping, brushing, curtain coating, and drawdown applicators.
The curable compositions of the present invention cure rapidly upon exposure to moisture or to exposure to temperatures greater than 70xc2x0 C. in the absence of moisture. The moisture level is not critical to achieving cure although the cure rate is dependent upon the level of moisture in combination with other factors such as the thickness of the applied curable composition, the catalyst level, and the catalyst type. Suitable moisture levels are 1% or greater relative humidity at temperatures of 0xc2x0 C. and higher, preferably 5% or greater relative humidity at temperatures of 0xc2x0 C. and higher, and most preferably 10% or greater relative humidity at temperatures of 0xc2x0 C. and higher. Moisture levels below 5% are sufficient to provide cure although the cure rate may be insufficient for many applications. The curable composition cures at ambient temperature as well as both higher and lower temperatures.
In another embodiment, photocurable compositions containing the alkoxysilane and/or acyloxysilane oligomers are prepared. The photocurable compositions include a photoinitiator which upon exposure to actinic radiation generates an acid or a base catalyst. The photoinitiators included in this invention are often referred to as photoacids or photobases and are chosen based on many factors including their photodissociation wavelength region, absorption strength, and cost. Preferred are photocatalysts which are soluble in the oligomer of the photocurable composition. Examples of photoacids and photobases are discussed in Progress in Polymer Science, 1996, Vol. 21, pp. 1-45 by Shirai and Tsunooka and include but are not limited to aryldiazonium salts, diarylhalonium salts, triarylsulfonium salts, nitrobenzyl esters, sulfones, O-acyloximes, and cobalt(III) amines. Blends of photoacid photoinitiators or blends of photobase initiators can be used to optimize the absorption region and the optical depth of the curable composition. The range of photoinitiator levels may be 0.1% to 20%, preferably 1% to 15%, and most preferably 2% to 10% photoinitiator weight based on weight of solids of the photocurable composition.
Cure is preferably effected by exposing the curable composition containing the photoinitiator to moisture during irradiation, after irradiation, or both. Effective moisture levels are similar to that discussed above.
The photocurable composition of this invention is irradiated with actinic radiation at various wavelengths. The optimum radiation wavelength region is determined by the absorption characteristics of the photoinitiator or blend of photoinitiators and is typically in the ultraviolet and visible wavelength region of 200 nm to 700 nm. The photocurable composition is stable when stored to minimize exposure to ambient light.
The photocurable composition can be prepared as described previously for the curable composition of the first aspect of this invention and can be applied by similar methods.
The curing composition and the photocurable composition of the present invention are useful in many applications including for example, protective films and coatings, barrier coatings, caulks, clear coats, sealants, automotive finishes, metal finishes, exterior coatings for concrete, masonry, and stone, binders for nonwoven fibers, adhesives, and surface treatments. Preferred surface treatments include sizing agents and coupling agents for hydroxyl functional surfaces such as glass, glass fibers, wood, aluminum and minerals such as talc, titanium dioxide, mica, wollostinite, silicates, and metal oxides.